1. Field of the Invention
This invention relates to a method of cleaning a porous body, and a process for producing a porous body, a non-porous film or a bonded substrate. More particularly, this invention belongs to a technical field of a production process in which a method of cleaning a porous body after anodization can be improved to form a non-porous film having a uniform thickness.
2. Related Background Art
In the following description, porous silicon is used as an example of the porous body.
Porous silicon was discovered by A. Uhlir and D. R. Turner in the course of their research related to the electrolytic polishing of single-crystal silicon biased to positive potential, in an aqueous hydrogen fluoride (hereinafter often simply xe2x80x9cHFxe2x80x9d) solution (i.e., hydrofluoric acid).
Since then, utilizing porous silicon""s high reactivity, studies were conducted related to its application to the step of interelement isolation that requires formation of a thick insulating material in a silicon integrated-circuit fabrication process, and a technique of FIPOS (full isolation by porous oxidized silicon) has been developed in which device elements are full isolated by a porous silicon oxide film (K. Imai, Solid-state Electron 24, 159, 1981).
Recently, in, e.g., Japanese Patent No. 2608351 and U.S. Pat. No. 5,371,037, there was proposed a technique in which a silicon epitaxial layer grown on a porous silicon substrate is bonded to the surface of an amorphous substrate or single-crystal silicon substrate, optionally via an oxide film, to obtain an SOI (silicon on insulator) substrate.
Besides, Japanese Patent Application Laid-Open No. 6-338631 discloses a technique that uses porous silicon as a light-emitting material such as what is called a photoluminescent material or an electroluminescent material.
Anodization is commonly used to form porous bodies.
As an example of the formation of porous bodies, an apparatus for producing porous silicon by subjecting a silicon substrate to anodization is shown in FIG. 18.
The apparatus or unit shown in FIG. 18 is the one disclosed in Japanese Patent Application Laid-Open No. 60-94737. This anodization apparatus comprises anodization baths 61 and 62 made of an HF-resistant material TEFLON (a trademark of Du Pont, U.S.A.) which are so provided as to hold between them a silicon substrate W as a treatment target. The baths 61 and 62 are provided with a negative electrode 63 and a positive electrode 64, respectively. The baths 61 and 62 have grooves in the sidewalls coming into contact with the silicon substrate W. In these grooves, sealing members such as O-rings 65 and 66 made of fluorine rubber are respectively fitted. Thus, the baths 61 and 62 holding the silicon substrate W are sealed with the O-rings 65 and 66. The baths 61 and 62 sealed in this way are filled with aqueous HF solutions 67 and 68, respectively.
Other anodization apparatuses have also been proposed.
Meanwhile, with regard to methods of cleaning porous semiconductor substrates after the anodization has been effected, an example is reported in Japanese Patent Application Laid-Open No. 10-64870, but it appears that very few other examples have been reported.
For the cleaning of porous bodies structurally having a high surface activity, liquid chemicals that are commonly used to remove organic matter, particularly metal deposits, such as an aqueous solution of sulfuric acid and hydrogen peroxide (hereinafter xe2x80x9cSPMxe2x80x9d), an aqueous solution of ammonia and hydrogen peroxide (hereinafter xe2x80x9cSC-1xe2x80x9d) and an aqueous solution of hydrochloric acid and hydrogen peroxide (hereinafter xe2x80x9cSC-2xe2x80x9d), cannot be used. Accordingly, a cleaning method has been proposed in which pure water is used with ultrasonic energy instead of the liquid chemicals to remove foreign matter adhering to porous layer surfaces, as disclosed in Japanese Patent Application Laid-Open No. 10-64870. FIG. 19 is a flow chart of the steps of such a cleaning method. A porous body, after it has been anodized in step STP1, is cleaned in step STP2 with pure water and ultrasonic energy, followed by drying in step STP3.
The above publication also discloses a method in which the porous layer surface is hydrophilicly treated with ozone water or hydrogen peroxide water and thereafter cleaned with pure water and ultrasonic energy.
However, while in cleaning porous semiconductors cleaning the surfaces is crucial, it is also important to remove the anodizing electrolytic solution that entered the fine the pores. This is because, even though the surface has been cleaned, the electrolytic solution (usually an aqueous HF solution with a concentration of 10% by weight to 50% by weight) remaining in the pores causes a change in structure of the porous body.
In addition, the HF that vaporizes gradually as HF gas from the interior of the pores may corrode the surrounding devices. Moreover, particles generated as a result of corrosion may contaminate the substrate.
Furthermore, since it takes time to replace the HF in the pores with pure water, cleaning with pure water must be carried out for a long time. In such a case, the porous body maybe crushed in pure water to generate a large quantity particles.
Such porous bodies are also preferably used in the production of bonded substrates utilized in SOI techniques.
FIG. 20 is a diagrammatic view to illustrate a process of producing a bonded substrate.
First, in step S1, a non-porous substrate 1 such as a single-crystal silicon wafer is prepared and its surface is made porous by anodization to form a porous layer 2 formed of single-crystal silicon.
Next, in step S2, the porous layer 2 is cleaned with pure water to wash away the foreign matter adhering to the porous layer or the electrolytic solution for anodization.
Subsequently, in step S3, a non-porous layer 3 formed of single-crystal silicon is epitaxially grown on the porous layer 2 by, e.g., CVD (chemical vapor deposition).
Then, in step S4, the surface of the non-porous layer is thermally oxidized to form an insulating layer 4.
In a subsequent step S5, the surface of the insulating layer 4 is bonded to a supporting base 5 prepared separately, to from a multi-layered structure in which the non-porous layer 4 is positioned inside.
In step S6, the non-porous portion of the substrate 1 is removed by grinding and by ion etching subsequent thereto.
Then, in step S7, the porous layer 2 thus uncovered is removed by etching with an aqueous solution containing HF and H2O2.
The surface of the non-porous semiconductor layer may optionally be smoothed by heat treatment in a reducing atmosphere containing hydrogen. Thus, a bonded substrate is obtained that has a thin semiconductor layer on an insulating layer formed on a supporting base. FIG. 21 illustrates the top surface of a bonded substrate obtained in this fashion. Reference numeral 12 denotes a notch.
However, upon observing the surface of the non-porous semiconductor layer thus formed, few circular spots 11 (hazes) that look optically different from their surrounding area are often seen. As a result of a careful observation of the circular spots 11, it has been found that these spots are due to the fact that the non-porous layer present on the insulating layer formed on the supporting base is locally thin. Namely, the non-porous layer proved to have locally caused microscopically uneven film thickness.
An object of the present invention is to provide a porous-body cleaning method by which the anodizing solution can be removed from the porous body without causing any change in porous structure of the porous body, even when cleaned for a short time, and to provide a process for producing a porous body.
Another object of the present invention is to provide a porous-body cleaning method that may hardly cause any corrosion of the surrounding devices, and a process for producing a porous body.
The cleaning method of the present invention is directed to cleaning a porous body formed by anodization, and is characterized by comprising the step of cleaning the porous body after the anodization is completed, with a cleaning solution containing at least one of an alcohol and acetic acid.
The porous-body production process of the present invention is characterized by comprising the step of subjecting a non-porous body to anodization and thereafter cleaning a porous body with a cleaning solution containing at least one of an alcohol and acetic acid.
Still another object of the present invention is to provide a process for producing a non-porous film and a bonded substrate that are free of uneven film thickness.
The above process of the present invention is characterized by a process for producing a non-porous film or a bonded substrate, the process comprising the steps of forming a porous layer by anodization, bonding to a supporting base a non-porous layer formed on the porous layer, and removing the porous layer, wherein the process further comprises the step of, after the anodization is completed, cleaning the porous layer with a cleaning solution containing at least one of an alcohol and acetic acid.
At first, the present inventors had thought that the haze described above occurs because of the conditions at the time of anodization or etching conditions that are not optimized when the porous layer is removed from the surface of the non-porous layer. However, any adjustment of these conditions was found not to affect substantially the uneven film thickness. Then, further studies made by the present inventors have revealed that the uneven film thickness is related to the treatment made after anodization.
Accordingly, in the present invention, the above cleaning method is employed in wet cleaning after anodization so that the non-porous layer of the bonded substrate may hardly cause uneven film thickness.